Detect client platform with Firebase

In my project I want to let admins access data from a web page and common users access from mobile app.

I read Firebase lets create a custom token to add fields to the standard token, but the web app (which should declare the privilege of the user) achieves this through node.js, which i don't know.

Does anyone know a simple way to distinguish between users by the platform they use? Can you suggest me any different approach?

Creating a custom token only on the Android/iOS device doesn't seem to me a good approach, since permits anyone to have admin privileges simply by skipping my apps.